Optical probes are currently used in control systems which continuously monitor the liquid level in vessels and containers. Such level sensing probes are used commonly in tank trucks and other enclosures where overfill protection is critical. One such probe, commercially available as models SP FU/MHC or SP FU/MHCF from Scully Electronic Systems, Wilmington, Mass. is illustrated in FIG. 1. The probe 40, comprises a cylindrical tube 44 having a light source 46, such as an infrared LED (IRLED), and a light detector 48, such as a photo transistor positioned at one end thereof. Light source 46 and light detector 48 are optically coupled to a 45.degree. glass prism 42 via mirror 45. The glass prism 42 is exposed at a second end of the tube, and is positioned perpendicularly to the tube axis 43. The liquid level is detected by aiming a pulsed beam of infrared light from the IRLED at one of the faces of the prism 42. If the prism tip is dry, the light is reflected off the other face of prism 42 and back into the optically matched photo transistor, completing the optical circuit and sending a "dry" signal to a controller 50 coupled to the probe. If the prism tip is wet, the light beam is refracted out of the first prism face, breaking the optical circuit and causing a "wet" signal to be sent to the controller 50.
Use of the probe, by itself, however presents some practical problems. First, the refractive properties of the prism tip are directly related to its physical integrity. Any damage to the prism tip will likely result in an inaccurate output signal from the probe. Accordingly, the prism tip must be physically protected from both the walls of the container and any solid contaminents which may be present in the fluid being measured. Second, the prism tip must be isolated from any stray light in the container which may be refracted through the prism, resulting in false readings. Third, light refracted out of the prism during the measurement process must be prevented from reentering the prism, resulting in false readings. Fourth, liquid may become trapped, due to the surface tension of the liquid, between the probe and any adjacent surfaces, resulting in false readings. Finally, the probe itself should be electrically grounded to ensure the integrity of the signal produced thereby.
Accordingly, it is an object of the present invention to provide a shield which physically protects the prism of an optical probe.
Another object of the present invention is to provide a prism shield which prevents stray light from the vessel interior from entering the prism.
A further object of the present invention is to provide a prism shield which prevents light refracted out of the prism from being reflected back through the prism in a wet condition.
Yet another object of the present invention is to provide a prism shield that ensures that all liquid trapped between the probe and itself will run off the probe.
Still another object of the present invention is to provide a prism shield which permits grounding of the probe through the shield itself.